Sublethal metabolic responses to contaminant mixture toxicity in Daphnia magna

Anthropogenic activity is increasing the presence of contaminants that enter waterways through wastewater effluent and urban and/or agricultural runoff, generally in complex mixtures. Depending on the mode of action of the individual contaminant within the mixture, toxicity can occur due to contaminants having similar or dissimilar modes of action. However, it is unknown how the metabolome responds to sublethal contaminant mixtures in the keystone genus Daphnia. In the present study we examined D. magna metabolic responses to acute sublethal exposure of propranolol, carbamazepine, and perfluorooctanesulfonic acid (PFOS) as well as in binary (propranolol-carbamazepine, propranolol-PFOS, carbamazepine-PFOS) and tertiary mixtures (carbamazepine-propranolol-PFOS), all at 10% of the median lethal concentration of the population (LC50). The metabolome was measured using ¹ H nuclear magnetic resonance (NMR) and characterized using principal component analysis, regression analysis, and fold changes in metabolite relative to the unexposed (control) group. The averaged principal component analysis scores plots revealed that carbamazepine-PFOS and carbamazepine-propranolol-PFOS exposures were significantly different from the control treatment. After normalizing the toxicity of each contaminant, we found that some metabolites responded monotonically, whereas others displayed a nonmonotonic response with increasing toxicity units. The single contaminant exposures and 2 binary mixtures (propranolol-carbamazepine, and propranolol-PFOS) resulted in minimal changes in the identified metabolites, whereas the carbamazepine-PFOS and carbamazepine-propranolol-PFOS displayed increases in several amino acid metabolites and decreases in glucose. Overall, our results highlight the sensitivity of the metabolome to distinguish the composition of the contaminant mixtures, with some mixtures displaying heightened responses versus others. Environ Toxicol Chem 2018;37:2448-2457. © 2018 SETAC.